Which device samples energy from within a waveguide for measurement?

A directional coupler is designed specifically to sample energy from a waveguide for measurement purposes. It works by allowing a portion of the signal power traveling through the primary line, or waveguide, to be coupled into a secondary line or port. This sampling action makes it possible to measure parameters like signal strength or reflection without disturbing the main signal flow.

In contrast, an amplifier is used to increase the power of a signal, and while it can be part of a measurement setup, it does not directly sample the energy for measurement. A power divider's primary function is to split an input signal into multiple output signals with equal power distribution, not to sample energy for measurement. A demodulator is used to extract information from modulated signals, but it does not deal with sampling energy from a waveguide for measurement purposes. Therefore, the directional coupler is the most appropriate choice for the task of sampling energy within a waveguide.